Probe Device for a Metrology Instrument and Method of Fabricating the Same

ABSTRACT

A method of producing a probe device for a metrology instrument such as an AFM includes providing a substrate having front and back surfaces and then forming an array of tip height structures on the first surface of the substrate, the structures having varying depths corresponding to selectable tip heights. The back surface of the substrate is etched until a thickness of the substrate substantially corresponds to a selected tip height, preferably by monitoring this etch visually and/or monitoring the etch rate. The tips are patterned from the front side of the wafer relative to fixed ends of the cantilevers, and then etched using an anisotropic etch. As a result, probe devices having sharp tips and short cantilevers exhibit fundamental resonant frequencies greater than 700 kHz or more.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with United States government support awarded bythe following agency: NIST/ATP (Award #70NANB4H3055). The United Stateshas certain rights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to probe devices for metrologyinstruments such as atomic force microscopes (AFMs), and morespecifically a method of producing a probe devices that allows forprecise and repeatable control over cantilever length, tip mass and tipheight so as to enable fast scanning AFM operation.

2. Discussion of the Prior Art

Several probe-based metrology instruments monitor the interactionbetween a cantilever-based probe and a sample to obtain informationconcerning one or more characteristics of the sample. For example,scanning probe microscopes (SPMs), including atomic force microscopes(AFMs), typically characterize the surface of a sample down to atomicdimensions by monitoring the interaction between the sample and a tip onthe cantilever probe. By providing relative scanning movement betweenthe tip and the sample, surface characteristic data can be acquired overa particular region of the sample, and a corresponding map of the samplecan be generated.

The probe of the typical AFM includes a small cantilever which has afixed end extending from a base, a sharp probe tip attached to the freeend of the lever, generally opposite the base. As discussed furtherbelow, the physical properties of the probe greatly impact the scanspeed at which the AFM may be operated. In operation, the probe tip isbrought very near to or into contact with a surface of a sample to beexamined, and the deflection of the cantilever in response to the probetip's interaction with the sample is measured with a deflectiondetector, such as an optical lever system, an example of which isdescribed in Hansma et al. U.S. Pat. No. RE 34,489. The probe may bescanned over a surface using a high-resolution three axis scanner actingon the sample support and/or the probe. The instrument is thus capableof creating relative motion between the probe and the sample whilemeasuring the topography, elasticity, or some other surface property ofthe sample as described, e.g., in Hansma et al. U.S. Pat. No. RE 34,489;Elings et al. U.S. Pat. No. 5,266,801; and Elings et al. U.S. Pat. No.5,412,980.

AFMs may be designed to operate in a variety of modes, including contactmode and oscillating mode. In contact mode operation, the microscopetypically scans the tip across the surface of the sample while keepingthe force of the tip on the surface of the sample generally constant.Some AFMs can at least selectively operate in an oscillation mode ofoperation such as TappingMode™ (TappingMode is a trademark of VeecoInstruments, Inc.) operation. In TappingMode™ operation the tip isoscillated, typically at or near a resonant frequency of the cantileverof the probe. The amplitude or phase of this oscillation is keptconstant during scanning using feedback signals, which are generated inresponse to tip-sample interaction. As in contact mode, these feedbacksignals are then collected, stored, and used as data to characterize thesample.

Regardless of their mode of operation, AFMs can obtain resolution downto the atomic level on a wide variety of insulating or conductivesurfaces in air, liquid or vacuum by using piezoelectric scanners,optical lever deflection detectors, and very small cantileversfabricated using photolithographic techniques. Because of theirresolution and versatility, AFMs are particularly important measurementdevices in many diverse fields including with particular application inconnection with the present preferred embodiments semiconductormanufacturing.

A scanning probe microscope, such as an atomic force microscope (AFM)operates by providing relative scanning movement between a measuringprobe and a sample while measuring one or more properties of the sample.A typical AFM system is shown schematically in FIG. 1. An AFM 10employing a probe device 12 including a probe 14 having a cantilever 15.Probe 14 is often a microfabricated cantilever with an integrated tip17. A scanner 24 generates relative motion between the probe 14 andsample 22 while the probe-sample interaction is measured. In this wayimages or other measurements of the sample can be obtained. Scanner 24is typically comprised of one or more actuators that usually generatemotion in three orthogonal directions (XYZ). Often, scanner 24 is asingle integrated unit that includes one or more actuators to moveeither the sample or the probe in all three axes, for example, apiezoelectric tube actuator. Alternatively, the scanner may be anassembly of multiple separate actuators. Some AFMs separate the scannerinto multiple components, for example an XY scanner that moves thesample and a separate Z-actuator that moves the probe.

In a common configuration, probe 14 is often coupled to an oscillatingactuator or drive 16 that is used to drive probe 14 at or near aresonant frequency of cantilever 15. Alternative arrangements measurethe deflection, torsion, or other motion of cantilever 15.

Commonly, an electronic signal is applied from an AC signal source 18under control of an SPM controller 20 to cause actuator 16 (oralternatively scanner 24) to drive the probe 14 to oscillate. Theprobe-sample interaction is typically controlled via feedback bycontroller 20. Notably, the actuator 16 may be coupled to the scanner 24and probe 14 but may be formed integrally with the cantilever 15 ofprobe 14 as part of a self-actuated cantilever/probe.

Often a selected probe 14 is oscillated and brought into contact withsample 22 as sample characteristics are monitored by detecting changesin one or more characteristics of the oscillation of probe 14, asdescribed above. In this regard, a deflection detection apparatus 25 istypically employed to direct a beam towards the backside of probe 14,the beam then being reflected towards a detector 26, such as a fourquadrant photodetector. Note that the sensing light source of apparatus25 is typically a laser, preferably a visible or infrared laser diode.The sensing light beam can also be generated by other light sources, forexample a He—Ne or other laser source, a superluminescent diode (SLD),an LED, an optical fiber, or any other light source that can be focusedto a small spot. As the beam translates across detector 26, appropriatesignals are transmitted to controller 20, which processes the signals todetermine changes in the oscillation of probe 14. In general, controller20 generates control signals to maintain a relative constant interactionbetween the tip and sample (or deflection of the lever 15), typically tomaintain a setpoint characteristic of the oscillation of probe 14. Forexample, controller 20 is often used to maintain the oscillationamplitude at a setpoint value, As, to insure a generally constant forcebetween the tip and sample. Alternatively, a setpoint phase or frequencymay be used.

A workstation is also provided, in the controller 20 and/or in aseparate controller or system of connected or stand-alone controllers,that receives the collected data from the controller and manipulates thedata obtained during scanning to perform point selection, curve fitting,and distance determining operations. The workstation can store theresulting information in memory, use it for additional calculations,and/or display it on a suitable monitor, and/or transmit it to anothercomputer or device by wire or wirelessly. The memory may comprise anycomputer readable data storage medium, examples including but notlimited to a computer RAM, hard disk, network storage, a flash drive, ora CD ROM. Notably, scanner 24 often comprises a piezoelectric stack(often referred to herein as a “piezo stack”) or piezoelectric tube thatis used to generate relative motion between the measuring probe and thesample surface. A piezo stack is a device that moves in one or moredirections based on voltages applied to electrodes disposed on thestack. Piezo stacks are often used in combination with mechanicalflexures that serve to guide, constrain, and/or amplify the motion ofthe piezo stacks. Additionally, flexures are used to increase thestiffness of actuator in one or more axis, as described in copendingapplication Ser. No. 11/687,304, filed Mar. 16, 2007, entitled“Fast-Scanning SPM Scanner and Method of Operating Same.” Actuators maybe coupled to the probe, the sample, or both. Most typically, anactuator assembly is provided in the form of an XY-actuator that drivesthe probe or sample in a horizontal, or XY-plane and a Z-actuator thatmoves the probe or sample in a vertical or Z-direction.

At present, the broadening use of SPM has demanded greater performanceover a wider range of applications. For example, AFM metrology isincreasingly being utilized in semiconductor fabrication facilities,primarily due to recent developments in automated AFM tools able toacquire sample measurements with higher throughput, such as theDimension® line of AFMs offered by Veeco Instruments Inc. These toolsare able to provide a variety of sub-nanoscale measurements, thereforemaking AFM a viable tool for measuring, for example, “criticaldimensions” of device features such as trenches and vias.

No matter the application, a significant limitation to AFM performanceis often the speed at which the AFM can scan the sample. As mentionedpreviously, the construction of the probe device significantly impactsscan speed. Two primary characteristics of probe devices usable for fastscanning applications are a sharp tip and precise control overcantilever dimensions. Techniques are known for producing probes withsharp tips but they are typically low yield and often provide onlylimited control over the length of the cantilever which ideally ismaintained at less than about 50 microns.

More generally, to enable fast scanning in scanning probe microscopy,control must be maintained over the resonant frequency of the cantileveras well as its spring constant, while the damping characteristics of theprobe when oscillating must also be considered. These factors areprimarily controlled by geometric factors associated with the probesincluding length of the cantilever, width of the cantilever, cantileverthickness and tip height. As noted, to maintain high yield andperformance, precise control of this geometry should be maintained.

In this regard, techniques for producing silicon nitride cantileverswith integral sharp tips are known. For example, in U.S. Pat. No.5,066,358 to Quate et al. describes a technique for producing siliconnitride probe devices. However, according to the Quate et al. technique,it is difficult to scale to small cantilevers with precise control overthe cantilever length, as well as the mass and the height of the silicontip. More particularly, in processes such as those disclosed in the '358patent, as well as in U.S. Pat. Nos. 5,021,364 and 5,811,017,electrochemical etch stops must be used in conjunction with a heavilydoped silicon or silicon-on-insulator wafers. This is due to the factthat when the required electrochemical wet etch is performed (FIG. 4 ofthe '358 patent), some structure is required to halt the etch whenforming the cantilever (36 in the '358 patent). Doped silicon providesthe appropriate structure, thus allowing the silicon to remain intactwith the backside etch.

However, there are significant drawbacks to using either heavily dopedsilicon or silicon-on-insulator wafers. Problems with prior arttechniques include high cost, lower production yields, stress bending ofcantilevers, and rough backside surfaces that interfere with successfuluse of the optical lever technique. Many prior art techniques also haveissues with errors associated with mask alignment, especially frontsideto backside alignment. These issues create a typical manufacturingtolerance for cantilever length and/or tip position of roughly ±5 μm.This manufacturing tolerance makes it difficult to manufacture smallcantilevers, for example smaller than 50 μm, or worse smaller than 10μm, with sufficient accuracy concerning its cantilever spring constant,resonant frequency and tip offset from the free end of the cantilever.

Tip formation is also less than ideal with these known techniques,exhibiting undesirable variations in tip height and tip sharpness. As aresult, conventional techniques do not exist that produce shortcantilevers through parallel processing on the wafer scale with highyield sharp tips (e.g., tip radius less than 20 nm, for example) and tipheights less than about 4 microns.

As a result, the field of atomic force microscopy was in need of amicrofabrication process that produces an AFM probe having an integraltip, and provides precise control over cantilever length to produceprobe devices having cantilevers with lengths less than 50 μm, andpreferably less than 40 microns and more preferably less than 10microns.

SUMMARY OF THE INVENTION

The preferred embodiments are directed to a process of producing smallcantilevers for high speed AFM scanning with high production yield, andwithout the limitations of known techniques. The preferred techniquesavoid using an electro-chemical etch to define tip height and thus donot require highly doped silicon or SOI wafers to produce the probedevices. Control over cantilever length in comparison to known processes(e.g., about +/−5 microns) is greatly improved, with the presenttechniques being capable of accuracy in the range of about +/−1 micron.Moreover, precise control is maintained over silicon tip mass and tipheight. Short cantilevers can thus be more readily and reliably producedwith high yield. In that regard, yield is further facilitated byproviding patterned holding tabs that minimize unintentional dislodgingof the resultant thin levered prove devices from the substrate.

According to one aspect of the preferred embodiment, a probe device fora probe microscope includes a base, as well as a cantilever made ofsilicon nitride. The cantilever has a fixed end and a free end with thefree end supporting a silicon tip positioned within about 5 μm of thefree end. In this case, the cantilever has a length less than about 50μm, while a height of the tip is less than about 4 μm with the effectivetip radius being less than about 20 nm.

In another aspect of this embodiment, the fundamental resonant frequencyof the cantilever is greater than 500 kHz.

In a further aspect of this embodiment, the quality factor Q of thecantilever in air is less than about one-hundred when the tip isinteracting with a sample.

According to yet another aspect of this embodiment, the height of thetip is substantially determined by monitoring an etch on one side of awafer. The etch reveals at least one of a series of thickness monitorfeatures formed on a side of the wafer opposite the one side.

In a still further aspect of this embodiment, the etch is terminatedonce a thickness of the wafer substantially corresponds to a selectedtip height. In this case, the etch is terminated by visually monitoringthe etch.

According to another aspect of this preferred embodiment, the etch isterminated based on an etch rate associated with the etch.

According to a still further aspect of this embodiment, the tip ispositioned by measuring, during fabrication, a distance between a tippattern and the fixed end. The fixed end of the cantilever is typicallyformed as a result of the backside etch used to reveal tip heightmonitor features. Importantly, the fixed end of the cantilever isvisible from the opposite side of the wafer (the front side), thusallowing the distance between the tip pattern and the fixed end to bemeasured.

In a further aspect of this embodiment, the probe device is formed froma silicon wafer. Notably, the silicon wafer may be a bulk single crystalsilicon wafer.

In another aspect of this preferred embodiment, a thickness of thecantilever is less than about 2 microns, with a tolerance of about 0.1micron.

According to another aspect of this embodiment, the thickness is lessthan about 1 micron. Moreover, the thickness is substantially uniformand the tolerance can be maintained among a plurality of probe devices.Further, the plurality of probe devices may include probe devices formedfrom different wafers.

According to another preferred embodiment, a probe device for a probemicroscope includes a base and a cantilever made of silicon nitride. Thecantilever has a fixed end and a free end and supports a silicon tippositioned within about 5 μm of the free end. In this embodiment, thetip height is less than about 4 μm, the effective tip radius is lessthan about 20 nm, and the cantilever has a fundamental resonantfrequency of greater than about 500 kHz. Ideally, the quality factor Qof the cantilever in air is less than about 100 when the probe tip isinteracting with a sample.

According to another aspect of this preferred embodiment, the resonantfrequency is greater than about 700 kHz.

In yet another aspect of the preferred embodiment, the resonantfrequency is greater than about 5 MHz.

According to a still further aspect of the preferred embodiments, alength of the cantilever is less than about 50 μm with a precision lessthan about +1-5 μm.

In a still further aspect of the preferred embodiments, the cantileverlength is less than about 50 μm with a precision less than about +/−1μm.

According to a still further aspect of the preferred embodiments, thetip height is substantially determined by monitoring an etch on one sideof a wafer, and wherein the etch reveals at least one of a series of tipheight monitor features formed on a side of the wafer opposite the oneside. Here, the etch is terminated once a thickness of the silicon wafersubstantially corresponds to a selected tip height.

In yet another aspect of the preferred embodiments, a method forfabricating a probe for a probe microscope includes providing a bulksingle crystal silicon wafer, etching a region of the silicon wafer to adesired thickness and then depositing silicon nitride on the backside ofthe etched silicon region to provide material to form the lever.Thereafter, the tip is patterned and etched from the etched siliconregion, and then the cantilever is patterned and etched from the siliconnitride. The cantilever may have a length of less than about 50 μm andthe tip may be positioned within about 5 μm of the free end of thecantilever. The resultant tip is sharp, with an effective radius lessthan about 20 nm.

According to a still further aspect of the preferred embodiments, thetip has a height less than about 4 μm.

In yet another aspect of the preferred embodiment, a tip height monitorfeature is formed to control the etching step.

According to another aspect of this preferred embodiment, the tip heightis controlled by monitoring a back side etch of the wafer.

According to a still further aspect of the preferred embodiments, themonitoring step includes at least one of monitoring etch rate andvisually inspecting the wafer.

In another aspect of this embodiment, during fabrication, the tip ispositioned by measuring a distance between a tip pattern and the fixedend.

In yet another aspect of the preferred embodiment, the probe device isone of an array of probe devices fabricated from the substrate and theyield is greater than about 90%.

According to another aspect of this preferred embodiment, the siliconnitride is supported by a thin film. The thin film may be an oxide.

In yet another aspect of the preferred embodiment, the thickness of thesilicon nitride is less than about 2 microns with a tolerance of about0.1 micron.

According to a still further aspect of this preferred embodiment, probedevice yield is maintained greater than about 90%.

In yet another aspect of the preferred embodiment, the tip is oxidationsharpened.

According to another aspect of this preferred embodiment, the tip ispyramid-shaped.

In yet another aspect of the preferred embodiment, a probe deviceincludes a base etched from a bulk silicon substrate/wafer and acantilever made of silicon nitride. The cantilever has a fixed end and afree end, with a tip being substantially supported by the free end. Inthese embodiments, the cantilever has a length that is less than about50 microns which can be produced while maintaining a precision less thanabout +/−5 microns, and even less than +/−1 micron.

In yet another aspect of the preferred embodiment, a method offabricating a probe device for a metrology instrument includes producingat least three probe devices each from a different substrate and eachhaving a length less than about 50 microns, wherein a standard deviationof the lengths is less than about 0.5 micron.

In yet another aspect of the preferred embodiment, a probe device for ascanning probe microscope includes a base, a cantilever extending fromthe base at a substantially fixed end. The cantilever also has a freeend, while a tip extends substantially orthogonally from the free end.Ideally, the tip can be maintained within about 50 microns of thesubstantially fixed end, with a tolerance of about plus/minus 2 microns.

These and other features and advantages of the invention will becomeapparent to those skilled in the art from the following detaileddescription and the accompanying drawings. It should be understood,however, that the detailed description and specific examples, whileindicating preferred embodiments of the present invention, are given byway of illustration and not of limitation. Many changes andmodifications may be made within the scope of the present inventionwithout departing from the spirit thereof, and the invention includesall such modifications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a prior art AFM, appropriately labeled“Prior Art”;

FIG. 2 is a top plan view of a semiconductor wafer for producing anarray of probe devices operable in a metrology instrument according tothe present invention, illustrating a series of monitor or tip heightfeatures patterned on a front side of the wafer on opposed ends of aperimeter of the wafer;

FIG. 3 is a top plan view of the tip height features shown in FIG. 2,also illustrating in phantom a backside reveal feature patternedperpendicularly to the front side monitor features;

FIG. 4A is a cross-sectional view of the wafer of FIG. 3, taken alonglines 4A-4A and illustrating an anisotropic etch of the monitor featuresto form V-grooves;

FIG. 4B is a cross-section view of the wafer of FIG. 3 taken along lines4B-4B, illustrating an anisotropic backside etch;

FIGS. 5A-5D are broken away side elevational views of a wafer beingprocessed into a probe according to the present invention;

FIG. 6 is a top plan view of a probe device produced according to thesteps shown in FIGS. 5A-5E;

FIG. 7A is an isometric image of a probe device produced according tothe present invention;

FIG. 7B is a side-elevational image of the probe device of FIG. 7A;

FIG. 8 is a flow chart illustrating a method of producing a probe deviceaccording to the present invention;

FIG. 9 is a schematic top plan view of a backside used to form probedevices with holding tabs sufficient to support the thin cantileveredprobe devices in the wafer;

FIG. 10A is a front side image of a probe device fabricated with holdingtabs, as illustrated in FIG. 9; and

FIG. 10B is a back side image similar to FIG. 10A, illustrating theholding tabs formed using an anisotropic etch.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments are directed to a micro-fabrication techniquein which probe devices are lithographically patterned and etched from asubstrate such as silicon ideally bulk single crystal silicon wafers.The resultant probe devices are particularly useful for high-speedmetrology applications, particularly using AFM, given that the processyields probes having high aspect ratio tips and short cantilevers, andthus large resonant frequencies. According to the invention, the yieldof useable probe devices is greatly improved over known methods bymaintaining flexibility with regard to the type of substrate that may beemployed to produce the probe devices (e.g., the substrate can be bulksilicon that is lightly doped or undoped), as well as providing a schemein which the tip of the probe device is aligned to the fixed end of thecantilever from the front side of the wafer, without moving the wafer,so as to allow precise control over cantilever length. Probe deviceshaving cantilever lengths less than 50 microns, and more ideally lessthan 10 microns can be repeatably and reliably produced, even acrossseveral wafers, with a standard deviation in the lengths of thecantilevers that are less than about 0.5 micron. Also, a scheme forcontrolling tip height is employed to produce tips with aspect ratios ina range of about 1 to 3, with tip heights less than about 4 microns, andeffective tip radii less than about 20 nm. As a result, probes havingfundamental resonant frequencies greater than about 500 kHz, and evengreater than 5 MHz, can be produced. To facilitate maximizing yield, therelatively thin probe devices produced according to the techniquesdescribed herein are held within the wafer using patterned and etchedholding tabs that allow ready removal of the probes, yet substantiallyprevent inadvertent separation of the probe devices from the wafer.

Turning initially to FIG. 2, a top view of a silicon substrate 50 fromwhich an array of probe devices usable for high speed scanning probemicroscopy can be fabricated is shown. A series or array 52 of tipheight or membrane thickness monitor windows 54 is patterned on at leasta portion of the substrate (opposed ends in this case) to not onlyprovide a way to monitor and substantially define a selected probe tipheight, as described further below, but also to align the front and backsides of the wafer during processing. These monitor windows are shown oneither side of the perimeter of wafer 50, with more or less windowsbeing required based on the uniformity of the membrane etching processthat is described below. Notably, controlling probe tip heightfacilitates achieving large fundamental resonant frequencies and allowsthe probe device manufacturer to manage squeeze film damping between theprobe and sample during SPM operation.

Referring next to FIG. 3, the array 52 of monitor windows 54 shown inFIG. 2 is used to substantially define tip height in the following way.Array 52 includes monitor windows 54 having varying widths (in thiscase, increasing from left to right) that are patterned on the wafer forsubsequent etching of the wafer at those locations. In that regard, asthe features are etched (preferably using an anisotropic etch), a seriesof substrate or membrane thickness monitor features or tip heightstructures in the shape of V-grooves are formed in the siliconsubstrate, with the V-grooves reaching a depth that is dependent on thewidth of the patterned monitor feature, as explained immediately below.V-grooves result due to the lattice structure of the silicon. Morespecifically, the anisotropic etch terminates on particular planes ofthe silicon, the (111) planes, in this case. Once the (111) planes arereached for the corresponding end points (width-wise) of the patternedfeature, the etch terminates and the depth of the feature (i.e.,V-groove) is defined. A backside etch is then performed to expose aselected one of the V-grooves and thus produce a wafer membrane that issubsequently used to fabricate tips having a desired height, asdescribed further immediately below in connection with FIGS. 4A and 4B.

Referring to FIG. 4A, using conventional thin film deposition andlithography techniques membrane thickness monitor windows are patternedon the wafer. In one embodiment, starting with a substrate such as asilicon wafer, a layer of oxide 60 (e.g., SiO₂) is deposited on thewafer, both sides, and then a layer of silicon nitride 62 is depositedon the oxide. The oxide and nitride act as etch masks for the followingprocess steps. Next, a photoresist is spun onto the oxide and nitridelayers and an appropriate mask (not shown) is used to pattern themonitor features on the wafer. More particularly, an array 52 of windows54 such as that illustrated in FIG. 3 are patterned onto the top surfaceof the substrate. Thereafter, the silicon is etched anisotropicallythrough array 52 of windows 54 so that membrane thickness monitorfeatures 64-78 in the shape of V-grooves are formed as the etchterminates on the (111) planes of the silicon crystal associated withthe end points of the width “w” of the features in FIG. 3. Again, theV-grooves have known depths directly related to the widths of the tipheight features.

Next, large windows 56 (FIG. 3) corresponding to the one or more arrays52 are patterned on the backside of the wafer. The edges 80, 82 of thesewindows are shown schematically in FIG. 4A. The large windows on thebackside are correspondingly aligned to one or more of the membranethickness monitor arrays 52 using standard alignment features andstandard front to backside alignment equipment, for example, an alignerthat employs a frontside-backside aligner from the Karl Suss Co. Inparticular, the major axis of window 56 is positioned substantiallyperpendicular to the major axes of front side windows 54. This alignmentis accomplished so that window 56 accommodates an etch through the waferdirectly below membrane thickness monitor array 52, as shown in FIG. 4B.

More particularly, in process, the back side etch is monitored visuallyfrom the front side, typically using illumination from the front or backsides. The etch is stopped when the V-groove having a depthcorresponding to a selected desired thickness d of monitor membrane 73(which is directly related to tip height) breaks through to the backsideof the wafer, as shown in FIG. 4B. Alternatively, the timing at whichdeeper V-grooves (for instance, V-groove 68 for desired V-groove 72)break through can be used to predict the appropriate time to stop theetch, i.e., an etch rate can be determined based on visual monitoring ofV-groove break through and using that information to determine aboutwhen to terminate the etch.

At the same time monitor membrane(s) 73 is being formed, devicemembranes 115 (shown in FIG. 5A) of the probes of the array of probesbeing fabricated are etched with a thickness that is substantially thesame as the thickness “d” of monitor membrane 73. Again, it is thisthickness d of device membrane 115 that is used to control the AFM probetip height.

This formation of the probe device is described in further detail andillustrated schematically in FIGS. 5A-5D. Referring initially to FIG.5A, layers of silicon dioxide 102 and silicon nitride 104 remain on thewafer 100, both front and back sides, once the monitor membrane 73 isformed to a desired thickness d. In producing membrane 73 with the backside etch, a structure 112 also results. Structure 112 forms the base ofthe probe device, described further below, which also substantiallydefines the location of a fixed end 116 of what ultimately will becomethe cantilever of each probe device.

At this point, a layer of silicon nitride 122 is deposited on thesilicon substrate, front and back sides 101, 103, the nitride 122ultimately being processed to form the cantilever. The formation ofnitride layer 122 is important in that the thickness of the layerdetermines the thickness of the cantilever which as noted earlier is afactor in determining the performance characteristics of the probedevice, including its fundamental resonant frequency. By controlling thethickness of the nitride deposition, the cantilever can be made thinnerto facilitate a more optimum probe device resonant frequency, thusallowing the probe, and the AFM as a whole, to operate at greaterspeeds. Nitride thickness may be monitored using any thin filmmeasurement tool, including, for example, an ellipsometer or anothersimilar tool, such as optical tools that measure reflectivity.Alternatively, fourier transform spectroscopy (FTIR), which employsinfrared interferometry to measure the thickness of the film, may beemployed.

To deposit the nitride 122, low pressure chemical vapor deposition(LPCVD) may be used. LPCVD advantageously provides a nitride layer 122that is low stress, thereby minimizing the adverse affects inherent toprior SPM probes fabricated, for example, with high stress siliconlevers. High stress silicon levers most often require that the stress becompensated (to accommodate lever bending, for instance), thus addingfurther complexity to the design, and ultimately limiting performance.

For the applications contemplated by the present invention,substantially even thickness is preferably maintained in a rangedependent on the length of the lever and the desired fundamentalresonant frequency. With the present silicon nitride levers, leverthicknesses less than two microns are desired, and thicknesses less thanone micron are possible, with a corresponding tolerance of about 6% orabout 0.1 micron. This precise control over lever thickness furtherfacilitates yield, with the present embodiments being able to maintainuniform cantilever thickness within the stated tolerances across awafer, as well as between wafers.

After the deposition of the silicon nitride layer 122, a support film orlayer 121 (e.g., silicon dioxide, a metal, a polymer) can be deposited,as an option. Support film 121 (a metal or a polymer, for instance) maybe deposited after the nitride 122 to give the membrane 115 morestructural integrity (or reduce stress of the nitride deposition) and/orto provide an etch stop for the formation of the cantilever. Byreinforcing the probe device in this way, the patterning of the tip(which occurs in the next step shown in FIG. 5B), can be re-run withoutcompromising the integrity of the device membrane 115. This patterningand re-patterning may be performed if it is determined that initialalignment will not yield cantilevers having the desired length. Idealpositioning of the patterned tip, including the improved accuracyachieved over known techniques, is described further below.

A tip feature 131 is then patterned onto the front side of the substrateas illustrated in FIG. 5B, in this case using global alignment marks.Then, as shown in FIG. 5C, an anisotropic etch is used on the front sideto form the generally pyramid-shaped tip. In this case, the width of thebase of the tip is substantially defined by the width of the patternimaged onto the front side silicon nitride 122 (as well as the siliconoxide 102 and nitride 104 therebelow).

More specifically, when performing the front side etch in this way, thegeometry of the resultant tip structure typically includes twopyramid-shaped structures 132, 133, disposed one on top of the other,tip-to-tip. Once the etch is complete, the top pyramid 133 breaks fromthe bottom pyramid 132 thus causing oxide 102, nitride 104, and nitride122 layers to be removed as well. This leaves a high aspect ratio tip132 having a height less than the membrane thickness d results. Thisformation of the tip using an anisotropic etch to terminate the etch onparticle facets of the substrate structure (i.e., crystal structure)facilitates producing tips with greater sharpness than known techniquesthat typically yield tips with rounded distal ends by patterning thesurfaces of the tip with conventional lithography techniques (asdescribed, for example, in the '358 patent).

When patterning the tip, especially for the high-speed AFM applicationscontemplated by the present invention, the location of this patterningis important to establishing overall device geometry, most notablycantilever length, 1_(c). The location of the tip relative to the fixedend of the lever substantially defines the effective length of the lever(except overhang, discussed below), and thus impacts the performancecharacteristics (e.g., resonant frequency) of the probe device. In thisregard, the user first patterns the tip based on global mask alignmentassociated with producing an array of probe devices. Once written,however, the distance between the patterned tip feature 131 and thefixed end 116 of the cantilever may be less than ideal for achieving thegoals of the preferred embodiments. Fortunately, the fixed end of thelever is generally visible from the front side 101. The fixed end of thecantilever, point 116 on the backside, is visible because the depositedsilicon nitride film is substantially transparent through the relativelythin silicon membrane, unlike those methods for producing probes havingsilicon levers. Notably, to further facilitate this viewing,illumination (front or backside) is typically employed.

With point 116 known, the distance between the fixed end 116 and thepatterned tip can be relatively accurately measured from the front side.Advantageously, cantilever length can then be substantially confirmedprior to etching the silicon to form the tip. In the event that thealignment between tip feature 131 and fixed end 116 is not acceptable,the probe device fabricator can strip tip feature 131 from the frontside and make appropriate position adjustments and re-write the featurein an attempt to produce the desired separation between the tip and thefixed end of the lever. This process can be repeated several times ifneeded to achieve the desired location of the patterned tip. As notedabove, however, multiple stripping and re-writing steps can compromisethe integrity of the device membrane, and thus that of the siliconnitride lever. As a result, and as also noted previously, an oxide layer(or a metal or another suitable film) may be used to reinforce the leverto resist membrane rupture during processing.

In sum, the tip masks can be positioned to pattern tips which canthereafter be measured, during probe device fabrication, to determinetheir location relative to the fixed end of the corresponding levers. Asa result, the present techniques can be used to repeatedly and reliablyproduce sub-50 micron probe device cantilevers 134 (FIGS. 5D and 6).Ideally, probe devices having cantilevers with lengths that are lessthan about 50 microns, and even less than about 10 microns, can berepeatedly produced, across wafers. Moreover, fundamental resonantfrequencies greater than 700 kHz, or greater than 1 MHz, or even greaterthan 5 MHz, can be realized with the present probe devices, with qualityfactors Q less than 100 in air when the tip is interacting with thesample.

Turning to FIG. 5D, the cantilever is then patterned and etched from thefront side, thus yielding, for example, the structure show schematicallyin FIG. 6. Referring more specifically to FIG. 5D, with the tip 132formed, the size and shape of the lever is patterned from the frontside. A free or distal end 135 of what will become lever 134 ispatterned with appropriate masking and defines an overhang or tipoffset, lt defined as the distance between end 135 and an axis throughthe apex of the tip and generally orthogonal to the lever. Tip offset,similar to the other probe device geometry discussed herein, isimportant to the overall performance substantially characteristics ofthe probe device and can significantly limit the speed at which the SPMcan reliably image a sample if made too large. Using the presenttechniques, tip offset, it, can be maintained within at least about 5 μmof the free end 132 of the fabricated lever. Notably, the tip ispyramid-shaped and the outside edge of the base of the pyramid may liesubstantially in line with the outer edge of the free end of the lever.As understood in the art, for optimum AFM operation, only the tip of theprobe device interacts with the sample, therefore, by maintaining tipoffset, 1 _(t). less than about 5 microns the chance that the free end135 of the probe device interacts with the sample during AFM operationis significantly reduced over previous techniques, which typicallyproduce probe devices with much larger, and less reliably controllable,overhang.

Ideally, the amount of overhang is controlled during probe fabrication.Notably, tip offset be reliably controlled using a stepper with afrontside alignment tool having at least 100 nm position accuracy.However, such equipment is expensive. Therefore, other conventionalalignment techniques may be used. According to one alternative, theresist used to pattern the lever may be a negative photoresist whichproduces a mask that is transparent when patterning the lever. In thisway, with tip 132 visible, the mask may be positioned so that the distalend 135 can be accurately placed so that offset, 1 _(t), can becarefully controlled.

An image of a cantilever produced according to the preferred embodimentsis shown in FIG. 7A in which a sub-30 micron cantilever length isproduced with a tip height less than about 10 microns. FIG. 7Billustrates the probe device of FIG. 7A from the side. Overall, by usingthe present techniques, cantilever size and shape can be precisely andrepeatedly controlled to produce high yield levers suitable for highspeed AFM operation. Moreover, probe devices having sub-50 microncantilever length can be maintained even among several wafers with astandard deviation of less than about 0.5 micron.

Turning to FIG. 8, a method 200 of producing these probe devices isshown. Initially, in Block 202 a substrate is provided. As highlightedpreviously, the substrate is typically a silicon substrate, however, oneadvantage of the preferred embodiments over known techniques is that thewafer can be a bulk silicon wafer, i.e., it does not have to be dopedsilicon or a costly silicon on insulator (SOI) wafer, due to the factthat the backside silicon etch is controlled by a particular monitoringtechnique. Again, because the method is sufficiently robust to use anundoped silicon substrate, several advantages are realized. Namely,front side alignment of the tip can be performed to repeatedly produceprobe devices having sub-50 micron cantilevers. And, high aspect ratiotips with sharp distal ends are realized given that an anisotropic etchis used to form the four-sided tip structure.

Finally, an additional highly reflective coating maybe employed on thebackside of the silicon nitride lever after removing the optionalsupporting layer to provide sufficient reflectivity for light impingingon the surface of the lever during AFM operation. Since no specialdoping is used, the method yields a smooth silicon surface upongenerating the silicon membrane. This provides a smooth silicon nitridesurface upon which the reflective layer can be deposited, this iscontrary to the highly doped silicon etch stop method which has a roughsurface after etching, yielding a rough surface upon which thereflective film is deposited.

Turning again to method 200, silicon oxide and silicon nitride are thendeposited on both sides of the wafer in Block 204. Next, an array of themembrane thickness monitor windows are patterned onto the nitride/oxideand etched from the front side in Block 206 to expose the silicon inthose areas. In Block 208, the large back side alignment reveal windowsare patterned onto the nitride generally opposite the front side array(e.g., at about ninety degrees to the front side monitor array) and thenetched to expose the backside silicon of the wafer to create revealwindows aligned with the array of front side windows, as describedabove.

At this point, in Block 210, the front or top side monitorscorresponding to the patterned windows of the array are etched so thatV-grooves with particular depths are formed with the (111) siliconfacets of the wafer exposed. In the meantime in Block 212, the back sidesilicon is etched to expose the large reveal windows. Moreover, becausethe rectangular reveal opening is aligned perpendicularly to the seriesof patterned V-groove or tip height features on the top side, sequentialthrough-holes are generated when the etching of the back side siliconreaches the bottom of the V-grooves formed on the front side. Bymonitoring the generation of the through-holes (and, for example, thesilicon etching rate, as described previously), the anisotropic backside etching step in Block 212 can be stopped to control the siliconmembrane thickness. As a result, the monitor membrane (73 in FIG. 4B) isformed. More specifically, by observing the progress of the etch andidentifying which membrane thickness monitor features (i.e., V-grooves)are revealed (FIG. 4B) the etch may be terminated. Again, the fabricatorcan stop the etch by either directly identifying the membrane thicknessmonitor feature directly related to the desired tip height, and/or bymonitoring the etch rate and determining the duration of the etchexpected to yield the desired silicon membrane thickness (i.e., toreveal the V-groove directly related to the desired tip height). Often,some combination of visually inspecting V-groove break-through andmonitoring etch rate is used to precisely control substrate thickness.Once the etch is terminated, a silicon membrane having a thicknesssufficient to pattern and etch the tip is produced.

Notably, concurrent with the formation of the monitor membrane (which,again, defines tip height), the support or base section of each probedevice of the array of probe devices being produced is formed. Moreover,the device membrane (115 in FIG. 5A) is also formed.

Next, in Block 214, silicon nitride is deposited over the entire wafer.Then an optional support layer (for example, silicon dioxide) may beformed on the wafer after the nitride deposition to act as an etch stopfor the lever and/or to provide support for the nitride, but it is notnecessary. Method 200 then includes, in Block 216, patterning the tipsof the probe devices. The patterned silicon is then etched in Block 218to form the tips as described above, the corresponding tips having apyramidal shape (4-sided), and a height less than the thickness of thesilicon membrane, due to the fact that the anisotropic etch terminateson the (111) facets of the silicon.

Method 200 then includes patterning and etching the silicon nitridecantilevers from the front side in Block 220. Once the probe devices areformed in this fashion, the tips may be sharpened in Block 222 prior tobeing removed from the wafer in Block 224. Oxidation sharpening canimprove the sharpness and effective aspect ratio of the tips, and thustheir performance, but it is not necessary. Again, with a sharper tip,the AFM has greater resolution (improved ability to image sub-nanometerfeatures), as well as an improved ability to obtain images of deepersample features such as trenches of semiconductor devices. Independentof whether the tip is sharpened, the corresponding performancecharacteristics (resonant frequency, etc.) of the resultant probedevices are maintained with the tips having an apex effective tip radiusless than about 200 nm, and ideally less than about 10 nm. Notably, inBlock 216, when the tips are patterned, the location of the tips can beselected based on a front-side measurement of the tip relative to afixed end portion of the lever, a region/point which can be identifiedfrom the front-side based on the fact that the silicon is substantiallytranslucent. This front-side positioning of where the tip is written, asdescribed previously, allows precise control over formation of thecantilever geometry. If after the tip is located the tip/fixed enddistance is determined to be non-ideal by the device fabricator the tipmay be re-written. Finally, as described above, a reflective metalcoating is provided on the back side of the lever in Block 223 tofacilitate the efficient reflection of a light beam directed at thelever by the AFM optical detection scheme, described in the Background.In the end, probe devices having sub-50 micron levers can be repeatedlyand reliably produced, even across wafers.

The corresponding fundamental resonant frequencies of probe devicesproduced in this fashion are maintained in a range of more than 750 kHz,and ideally more than 1 MHz, with fundamental resonant frequenciesgreater than 5 MHz being possible. Moreover, the corresponding qualityfactor Q of the cantilevers in free air is maintained at less than about100 in free air when the tip is interacting with the sample. For thepurposes of this application, “interacting with the sample” generallyrefers to the situation where the probe is close enough to a sample tosense at least the long range forces from the sample. In practice, thismay be when the apex of the tip is within roughly 100 nm of the samplesurface.

More particularly, in the preferred embodiments, the cantilever has aresonant frequency from roughly 700 kHz to more than 5 MHz. Depending onthe desired tip-sample interaction force and cantilever resonantfrequency, the cantilevers are typically 5-50 μm long by 3-20 μm wide,by 1-5 μm thick. For example, a cantilever with a length of 10 μm and awidth of 5 μm and a thickness of 0.5 um and a 2 um tall tip will have aresonant frequency of roughly 6 MHz and a spring constant around 20 N/m.A cantilever around 35 μm long with a width of 15 μm and a thickness of0.8 μm has a resonant frequency of roughly 800 kHz, with a springconstant around 6 N/m. In one embodiment, the applicants have builttrapezoidal cantilevers with a cantilever length of 35 μm, a base widthof 40 μm, tapering to an end width of 7 μm, with a 0.6 μm thickness anda 3 μm tall tip. These probes were coated with 5 nm of Ti and 40 nm ofgold (Au) to form a reflective coating. These probes have a resonantfrequency of 750 kHz and a spring constant of around 8 N/m.

The shape of the high bandwidth cantilever probes may be rectangular,triangular, trapezoidal or other arbitrary shape to achieve the desiredresonant frequency and spring constant. The dimension and shape can beoptimized for particular combinations of resonant frequency, springconstant and quality factor Q, for example by calculations includingformulas available in the literature and/or by computational fluiddynamics and/or experimentation. The quality factor Q depends not onlyon the viscous damping of the lever but also a “squeeze film” effect asthe tip and cantilever come close to the sample surface. This effect canbe optimized by controlling the shape of the cantilever and the heightof the tip. Taller tips move the bulk of the cantilever further from thesample surface and reduce the squeeze film effect, resulting in higher Qcantilevers. In some cases it is desirable to make shorter tips toreduce the Q and thus decrease the cantilever response time. Using shorttips around 1 μm tall, the applicants have made high frequencycantilevers with Q values of less than 50 in air with the tip close tothe sample surface.

Again, the cantilevers are generally fabricated with a sharp tip. Inthat regard, for most high resolution imaging applications a tip havingan end radius of <20 nm is desired. However, for some applications,especially force measurements on soft samples, a duller tip ispreferred.

Turning to FIGS. 9 and 10, due to the fact that the silicon membraneproduced according to the present invention is very thin (on the orderof several microns), it is useful during processing to provideadditional structure to support the probe devices within the wafer, thuspreventing the probe devices from accidentally detaching from the waferor otherwise being damaged. Because these high speed AFM probe devicesare high cost components, maintaining probe device yield high is apriority to the probe device fabricator. The present invention employsetch stop V-groove holding tabs which not only insure high yield bysubstantially insuring the probe devices remain in the wafer until thefabricator wishes to remove them, but also provide a mechanical weakpoint to break the probe die from the wafer frame. In this fashion, theyield can be maintained at greater than 90%.

With more particular reference to FIG. 9, a mask design 300 for abackside silicon etch to produce the holding tabs is shown. V-groovescorresponding to the holding tabs are formed in the regions 302, 304 ofthe mask, while the probe devices lie in mask regions 306 of the mask.FIG. 10A illustrates a top-view image 310 of a probe device 312 from thefront-side of the wafer, including a probe having a silicon nitridelever 314, once holding tabs 316, 318 are formed. Holding tabs 316, 318provide sufficient structure to secure probe die from being removed fromthe wafer.

Referring to FIGS. 9 and 10B, to allow the probe devices to be readilyremoved yet not compromise the integrity of the holding tabs 316, 318,V-grooves are formed at regions 302, 304 using a back side anisotropicetch, similar to the formation of the tip height thickness monitorfeatures described earlier. Once formed, as shown in FIG. 10B, theV-grooves 320 (with an inflection point 322 in the region of the tabs316, 318-FIG. 10A) allow the tabs 316, 318 to be selectively brokenalong the V-groove to allow the probe devices to be removed from thesubstrate 324.

More particularly, when formed with this backside etch, V-grooves 320have a depth that is significantly greater than the thickness of thesilicon membrane at the locations at which the probe devices are formed.In this way, holding tabs 316, 318 are able adequately secure theresultant probe devices to the wafer 309, yet also provide a weak pointat the V-groove discontinuity 322 to allow the probe device to beremoved from the wafer 324. Notably, though two holding tabs are shownin the figures, one may only be required.

Although the best mode contemplated by the inventors of carrying out thepresent invention is disclosed above, practice of the present inventionis not limited thereto. It will be manifest that various additions,modifications and rearrangements of the features of the presentinvention may be made without deviating from the spirit and scope of theunderlying inventive concept.

1. A probe device for a probe microscope, the probe comprising: a base;a cantilever made of silicon nitride and having a fixed end and a freeend; a silicon tip positioned within about 5 μm of the free end; andwherein the cantilever has a length less than about 50 μm, a height ofthe tip is less than about 4 μm, and the effective tip radius is lessthan about 20 nm.
 2. The probe device of claim 1, wherein thefundamental resonant frequency of the cantilever is greater than about700 kHz.
 3. The probe device of claim 1, wherein the quality factor Q ofthe cantilever in air is less than about 100 when the tip is interactingwith a sample.
 4. The probe device of claim 1, wherein the length has aprecision equal to less than about +/−5 μm.
 5. The probe device of claim1, wherein the height of the tip is substantially determined bymonitoring an etch on one side of a wafer, and wherein the etch revealsat least one of a series of thickness monitor features formed on a sideof the wafer opposite the one side.
 6. The probe device of claim 5,wherein the etch is terminated once a thickness of the etched wafersubstantially corresponds to a membrane thickness determined for aselected tip height.
 7. The probe of claim 1, wherein the tip height isless than 2 μm.
 8. The probe of claim 1, wherein the tip height is lessthan 1 μm.
 9. The probe device of claim 5, wherein the tip is positionedby measuring during fabrication a distance between a tip pattern and thefixed end.
 10. The probe device of claim 9, wherein the fixed end of thecantilever is formed by the etch on the one side of the wafer, the fixedend being visible from the opposite side of the wafer such that thedistance can be measured.
 11. The probe device of claim 5, wherein theprobe device is formed from a silicon wafer.
 12. The probe device ofclaim 11, wherein the silicon wafer is a bulk single crystal siliconwafer.
 13. The probe device of claim 1, wherein a thickness of thecantilever is less than about 2 microns with a tolerance of about 0.1micron.
 14. The probe device of claim 13, wherein the thickness is lessthan about 1 micron.
 15. The probe device of claim 13, wherein thethickness is substantially uniform and the tolerance can be maintainedamong a plurality of probe devices.
 16. The probe device of claim 15,wherein the plurality of probe devices include probe devices formed fromdifferent wafers.
 17. A probe device for a probe microscope, the probecomprising: a base; a cantilever made of silicon nitride and having afixed end and a free end; a silicon tip positioned within about 5 μm ofthe free end; and wherein the tip height is less than about 4 μm, theeffective tip radius is less than about 20 nm, and the cantilever has afundamental resonant frequency of greater than about 500 kHz, and aquality factor Q of the cantilever in air is less than about 100 whenthe probe tip is interacting with a sample.
 18. The probe device ofclaim 17, wherein the resonant frequency is greater than about 700 kHz.19. The probe device of claim 18, wherein, the resonant frequency isgreater than about 5 MHz.
 20. The probe device of claim 17, wherein alength of the cantilever is less than about 50 μm with a precision equalto less than about +/−5 μm.
 21. The probe device of claim 20, wherein alength of the cantilever is less than about 50 μm with a precision equalto less than about +/−1 μm.
 22. The probe device of claim 17, whereinthe tip height is substantially determined by monitoring an etch on oneside of a wafer, and wherein the etch reveals at least one of a seriesof tip height monitor features formed on a side of the wafer oppositethe one side, wherein the etch is terminated once a thickness of thesilicon wafer substantially corresponds to a selected tip height.
 23. Amethod for fabricating a probe for a probe microscope comprising thesteps of: providing a bulk single crystal silicon wafer; etching aregion of the silicon wafer to a desired thickness; depositing siliconnitride on the backside of the etched silicon region; patterning andetching a tip from the etched silicon region; patterning and etching acantilever from the silicon nitride; and wherein the cantilever has alength of less than about 50 μm and wherein the tip positioned withinabout 5 μm of the free end of the cantilever and the effective radius ofthe tip is less than about 20 nm.
 24. The method of claim 23, whereinthe tip has a height of less than about 4 μm.
 25. The method of claim24, wherein a tip height monitor feature is formed to control theetching step.
 26. The method of claim 24, wherein the tip height iscontrolled by monitoring a backside etch of the wafer.
 27. The method ofclaim 26, wherein the monitoring step includes at least one of a)monitoring an etch rate associated with the backside etch, and b)visually inspecting the wafer during the backside etch.
 28. The methodof claim 24, wherein, during fabrication, the tip is positioned bymeasuring a distance between a tip pattern and the fixed end.
 29. Themethod of claim 24, wherein the probe device is one of an array of probedevices fabricated from the substrate and the yield is greater thanabout 90%.
 30. The method of claim 21, wherein the silicon nitride issupported by a thin film.
 31. The method of claim 30, wherein the thinfilm is an oxide.
 32. The method of claim 30, wherein the thickness ofthe silicon nitride is less than about 2 microns with a tolerance ofabout 0.1 micron.
 33. The method of claim 32, wherein probe device yieldis maintained greater than about 90%.
 34. The method of claim 23,wherein the tip is oxidation sharpened.
 35. The method of claim 23,wherein the tip is pyramid-shaped.